1. Field of the Invention
The invention relates to a technical field of a constant-temperature type crystal oscillator (hereinafter referred to as constant-temperature type oscillator) using a crystal unit for surface mounting, and in particular, to a constant-temperature type oscillator in which the efficiency of heat conduction from heating chip resistors (hereinafter referred to as heating resistors) is enhanced.
2. Description of the Related Art
Constant-temperature type oscillators keep the operational temperatures of their crystal units constant. Thus, even when an ambient temperature is changed, the frequency stability can be improved. Therefore, the constant-temperature type oscillators are applied to wireless devices in communication facilities which are, for example, base stations. In recent years, in place of a traditional constant-temperature bath, on which heating coils are wound, heating resistors are used as a heat source so as to simplify a constant-temperature structure. Then, there is a constant-temperature type oscillator in which a crystal unit is formed for surface mounting, which is reduced in height size (see, for example, JP-A-2006-311496 and JP-A-2005-341191).
FIGS. 6A to 8 are diagrams for explanation of one example of a related art constant-temperature type oscillator. FIG. 6A is a cross-sectional view of the related art constant-temperature type oscillator, and FIG. 6B is a circuit diagram thereof. FIG. 7A is a cross-sectional view of a crystal unit thereof, FIG. 7B is a bottom view thereof, and FIG. 7C is a frequency-temperature characteristic diagram thereof. FIG. 8 is a bottom view of a circuit substrate thereof.
The constant-temperature type oscillator has a crystal unit 1 for surface mounting, respective circuit elements 4 forming an oscillator output circuit 2 and a temperature control circuit 3, and a circuit substrate 5 on which the circuit elements 4 including the crystal unit 1 are installed thereon (see FIGS. 6A to 6C). Then, the constant-temperature type oscillator is configured such that the circuit substrate 5 is held with lead wires 8 of a metal base 7 which is made airtight with glass 6, and these are covered with a metal cover 9 by resistance welding or the like.
With respect to the crystal unit 1, a crystal element 1A is housed in a case main body 10 which is formed to be concave, for example, and its opening end face is sealed up with a metal cover 11, for example, to hermetically encapsulate the crystal element 1A having an excitation electrode (not shown) and leading electrode (not shown) on both principal surfaces (see FIGS. 7A to 7C). The case main body 10 is made of laminar ceramic, for example, and has crystal terminals 12a electrically connected to the leading electrode of the crystal element 1A on a set of diagonal corners of the outer bottom face, for example, and has dummy terminals 12b on the other set of diagonal corners. The dummy terminals 12b are electrically connected to the metal cover via an electrically-conducting path including through electrodes 14 and the like, for example, and are connected to the ground potential.
The crystal element 1A is formed as, for example, an SC-cut crystal element or an AT-cut crystal element, and has the frequency-temperature characteristic that approximately 80° C. at the higher temperature side higher than or equal to 25° C. as room temperature is an extreme value, and the oscillating frequency varies according to a temperature in any cases of both of the SC-cut and AT-cut crystal elements. For example, in an AT-cut crystal element, the frequency-temperature characteristic shows a cubic curve (curve A in FIG. 7C), and in an SC-cut crystal element, the frequency-temperature characteristic shows a quadratic curve (curve B in FIG. 7C). Incidentally, frequency deviation Δf/f is plotted along the ordinate of the diagram, where f is a frequency at room temperature, and Δf is a frequency difference from the frequency f at room temperature.
The oscillator output circuit 2 includes an oscillating stage 2a serving as an oscillator circuit and a buffering stage 2b having a buffer amplifier or the like. The oscillating stage 2a is formed as a Colpitts type circuit having a voltage dividing capacitor (not shown) and transistor (not shown) for oscillation, that form a resonance circuit along with the crystal unit 1. Here, the oscillating stage 2a is formed as a voltage control type circuit having a voltage-controlled capacitative element 4Cv in an oscillatory loop, for example. In the drawing, Vcc is a power source, Vout is an output, and Vc is a control voltage.
In the temperature control circuit 3, a temperature sensing voltage Vt by a temperature sensing element (for example, thermistor) 4th and a resistor 4R1 is applied to one input terminal of an operational amplifier 40A, and a reference voltage Vr by resistors 4R2 and 4R3 is applied to the other input terminal. Then, a differential voltage between the reference voltage Vr and the temperature sensing voltage Vt is applied to the base of a power transistor 4Tr, and electric power from the power source Vcc is supplied to the chip resistors (hereinafter called heating resistors 4h) 4h serving as heating elements. Thereby, the electric power to the heating resistors 4h is controlled with a temperature-dependent resistance value of the temperature sensing element 4th, to keep the operational temperature of the crystal unit 1 constant. An operational temperature is to be approximately 80° C. which is a minimum value or a maximum value at room temperature or more, for example (see FIG. 8).
The circuit substrate 5 is made of, for example, glass epoxy as a base material, and a circuit pattern (not shown) is formed thereon, and the respective circuit elements 4 including the crystal unit 1 are installed on both principal surfaces thereof (see FIG. 6A). In this example, the crystal unit 1, and the heating resistors 4h, the power transistor 4Tr, and the temperature sensing element 4th in the temperature control circuit are installed on the bottom face of the circuit substrate 5. The crystal unit 1 is installed on the central area, and the heating resistors 4h, the power transistor 4Tr, and the temperature sensing element 4th are installed on the outer circumference thereof. However, here, the voltage-controlled capacitative element 4Cv which is temperature-dependent to greatly vary its characteristic, is installed on the outer circumference of the crystal unit 1. These crystal unit 1 and circuit elements 4 are covered with heat conducting resin 13.
Then, the other circuit elements 4 of the oscillator output circuit 2 and the temperature control circuit are installed on the top face of the circuit substrate 5. In this case, for example, a capacitor for adjusting oscillating frequency and the like are installed on the top face of the circuit substrate 5, which makes it easy to adjust oscillating frequency. Then, in particular, the respective circuit elements 4 of the oscillating stage 2a having an affect on an oscillating frequency are disposed on the top face of the circuit substrate 5 facing the area covered with the heat conducting resin 13.
However, in the constant-temperature type oscillator having the above-described configuration, the heating resistors 4h and the power transistor 4Tr are thermally coupled particularly to the crystal unit 1 with the heat conducting resin 13, meanwhile, the heat conductivity of the heat conducting resin 13 is less than or equal to 1/100 of that of metal such as, for example, gold (Au) or copper (Cu) of an electrically-conducting path as a circuit pattern, which results in the problem that the efficiency of heat conduction thereof is inferior to that of a crystal unit. Meanwhile, the heat conductivity of the heat conducting resin 13, for example, KE-3467 is 2.4, and those of Au and Cu serving as an electrically-conducting path are respectively 319 and 403.